JP7757685B2 - electric braking device - Google Patents

Description

The present invention relates to an electric braking device that generates braking force using the power of an electric motor.

Electric braking devices are known that generate braking force through the linear movement of a piston inside a cylinder powered by an electric motor. There are two types of electric braking devices: wet-type electric braking devices, which generate braking force by transmitting piston pressure to a friction member via brake fluid, and dry-type electric braking devices, which generate braking force by directly transmitting piston pressure to a friction member.

In such electric braking devices, if the electric motor loses power while generating braking force due to a power failure or other reason, the piston is pushed back. The impact caused when the piston hits the end of its linear motion range within the cylinder can potentially damage the durability of the components of the electric braking device. In response to this, Patent Document 1 describes an electric braking device equipped with a clutch mechanism to protect the components from such impact. The clutch mechanism equipped in the electric braking device in this document cuts off the power transmission path between the electric motor and the linear motion conversion mechanism when the piston is pushed back beyond a predetermined position within the cylinder.

DE 102018214188

Providing a clutch mechanism such as the one described above can protect the components of the electric braking device from the impact described above. However, such a clutch mechanism may not be feasible due to installation space and component costs.

In an electric braking device that solves the above problems, rotational motion generated by an electric motor is transmitted to a linear motion conversion mechanism. The linear motion conversion mechanism then converts that rotational motion into linear motion that drives a piston located inside a cylinder. This generates a braking force on the vehicle by pressing a friction part against a frictioned part that rotates with the vehicle's wheels. The electric braking device also includes a resistance generating part that increases the sliding resistance generated by the piston's movement when the piston moves backward from its initial position, thereby reducing the vehicle's braking force, compared to when the piston moves forward from its initial position, thereby increasing the vehicle's braking force. The initial position is the boundary between the piston's normally used position and its normally unused position in the electric braking device.

In the above-mentioned electric braking device, when braking force is normally generated, the piston is moved in a range in the forward direction from its initial position. If the electric motor loses power while generating braking force due to a power failure or other reason, the piston may be pushed backward from its initial position. As a result, the piston may move backward until it hits the backward end of its linear motion range, potentially causing an impact.

In contrast, with the electric braking device described above, when the piston moves backward from its initial position, the resistance generating section increases the sliding resistance that occurs as the piston moves, thereby reducing the piston's backward movement speed. Therefore, with the electric braking device described above, it is possible to mitigate the impact that occurs when the electric motor loses power while generating braking force.

1 is a cross-sectional view schematically illustrating a configuration of a first embodiment of an electric braking device. FIG. 3 is a cross-sectional view showing the state of the electric braking device when power loss occurs. FIG. 10 is a cross-sectional view of a cylinder and its surrounding area in an electric braking device according to a modified example of the first embodiment. FIG. 10 is a cross-sectional view of the periphery of an O-ring in an electric braking device according to a modified example of the first embodiment. 4 is a cross-sectional view showing a state of the O-ring and its surroundings in the electric braking device when power is lost. FIG. FIG. 10 is a cross-sectional view of the periphery of an O-ring in an electric braking device according to a modified example of the first embodiment. FIG. 11 is a partial cross-sectional view of a threaded portion of a linear motion conversion mechanism in an electric braking device according to a second embodiment. 8 is a cross-sectional view showing a state in which the piston has moved backward from the initial position in the threaded portion of the linear motion conversion mechanism of FIG. 7. FIG. 11 is a partial cross-sectional view of a threaded portion of a linear motion conversion mechanism in an electric braking device according to a modified example of the second embodiment. 10 is a cross-sectional view showing a state in which the piston has moved backward from the initial position in the threaded portion of the linear motion conversion mechanism of FIG. 9 . FIG. 11 is a cross-sectional view of a cylinder and its surroundings in an electric braking device according to a third embodiment. FIG. 3 is a cross-sectional view of the periphery of a sliding contact hole of the electric braking device.

(First embodiment)
An embodiment of an electric braking device will be described below with reference to Figures 1 and 2. The electric braking device 10 of this embodiment is configured as a device that generates braking force on wheels 27 of a vehicle.

<Configuration of electric braking device 10>
First, the configuration of an electric braking device 10 of this embodiment will be described with reference to Figure 1. The electric braking device 10 includes an electric motor 11, a rotational transmission mechanism 12, and a linear motion conversion mechanism 13. In the electric braking device 10 of this embodiment, the electric motor 11 is fixed to the outer periphery of a housing 19 that houses the rotational transmission mechanism 12 and the linear motion conversion mechanism 13.

The rotation transmission mechanism 12 is a mechanism that transmits the rotational motion of the electric motor 11 to the linear motion conversion mechanism 13. In this embodiment, a gear mechanism consisting of two gears is used as the rotation transmission mechanism 12. That is, the rotation transmission mechanism 12 in this embodiment has a first gear 15 fixed to the output shaft 14 of the electric motor 11 so as to rotate integrally with it, and a second gear 16 meshed with the first gear 15. The second gear 16 has more gear teeth than the first gear 15.

The linear motion conversion mechanism 13 converts the rotational motion of the electric motor 11, transmitted via the rotation transmission mechanism 12, into linear motion. In this embodiment, a ball screw mechanism having a screw shaft 17 and a nut 18 is used as the linear motion conversion mechanism 13. In the linear motion conversion mechanism 13 shown in Figure 1, the screw shaft 17 is connected to the second gear 16 so that it rotates integrally with the second gear 16. In this linear motion conversion mechanism 13, the screw shaft 17 corresponds to the rotating part that is rotated by the electric motor 11. The nut 18 corresponds to the linear motion part that moves linearly together with the piston 21, which will be described later.

The electric braking device 10 also has a cylinder 20 that houses a piston 21 for linear motion. In this embodiment, the inner wall of the housing 19 forms the cylinder 20. In addition to the piston 21, the cylinder 20 also houses a linear motion conversion mechanism 13. The piston 21 is connected to a nut 18 of the linear motion conversion mechanism 13 and moves within the cylinder 20 together with the nut 18. Therefore, the piston 21, which is provided inside the cylinder 20, is driven by the linear motion converted by the linear motion conversion mechanism 13. Meanwhile, within the cylinder 20, a fluid chamber 22, into which brake fluid is introduced, is defined by the piston 21. The volume of the fluid chamber 22 changes depending on the position of the piston 21 within the cylinder 20. In the following description, linear motion of the piston 21 in a direction that reduces the volume of the fluid chamber 22 is referred to as the advancement of the piston 21. Linear motion of the piston 21 in a direction that increases the volume of the fluid chamber 22 is referred to as the retreatment of the piston 21. Furthermore, in the following description, the direction in which the piston 21 moves forward within the cylinder 20 will be referred to as the forward direction F, and the direction in which the piston 21 moves backward will be referred to as the backward direction R.

A stopper 18A, which is a protrusion that projects in the backward direction R, is formed on the end of the nut 18 in the backward direction R. Inside the cylinder 20, the piston 21 can be retracted to a position where the stopper 18A of the nut 18 abuts against the second gear 16. In the following description, the position of the piston 21 inside the cylinder 20 at this time is referred to as the most retracted position.

The cylinder 20 has two ports, an input port 23 and an output port 24, which communicate between the inside and outside of the cylinder 20. The input port 23 is connected to a reservoir tank 25, which stores brake fluid. The output port 24 is connected to a wheel cylinder 26. In response to the generation of fluid pressure, the wheel cylinder 26 presses a brake shoe 26A against a brake disc 26B, which rotates with the wheel 27. The output port 24 is configured to maintain communication with the fluid chamber 22 regardless of the position of the piston 21 within the cylinder 20. In this embodiment, the brake shoe 26A corresponds to the friction part, and the brake disc 26B corresponds to the frictioned part.

On the other hand, when the piston 21 is in its most retracted position, the input port 23 is in communication with the fluid chamber 22. When the piston 21 moves forward a certain amount from its most retracted position, the opening of the input port 23 to the cylinder 20 is blocked by the piston 21, blocking communication with the fluid chamber 22. In the following description, the position of the piston 21 at which the input port 23 switches between being in communication with the fluid chamber 22 and being blocked is referred to as the initial position of the piston 21. Figure 1 shows the state of the electric braking device 10 when the piston 21 is in its initial position.

In this electric braking device 10, the linear motion conversion mechanism 13 converts the rotational motion of the electric motor 11, transmitted via the rotational transmission mechanism 12, into linear motion and transmits it to the piston 21, moving the piston 21 within the cylinder 20. When the piston 21 is positioned further forward in the forward direction F than its initial position, only the output port 24 is connected to the fluid chamber 22. Therefore, when the piston 21 moves forward from its initial position and presses the brake fluid in the fluid chamber 22, the brake fluid is sent to the wheel cylinder 26 through the output port 24. As a result, when fluid pressure is generated in the wheel cylinder 26, the brake shoe 26A is pressed against the brake disc 26B, generating a braking force on the wheel 27. The electric braking device 10 of this embodiment controls the braking force generated on the wheel 27 by moving the piston 21 within a range further forward in the forward direction F than its initial position and adjusting the fluid pressure in the wheel cylinder 26. In this embodiment, the position of the piston 21 further forward in the forward direction F than its initial position is the position in which the piston 21 is normally used. When controlling the braking force, the piston 21 is not moved in the backward direction R from its initial position. In other words, the position of the piston 21 in the backward direction R from its initial position is a position of the piston 21 that is not normally used. The initial position is the boundary position between the position of the piston 21 that is normally used and the position of the piston 21 that is not normally used.

<Resistance generating section>
An O-ring 28 is installed in the cylinder 20 of the electric braking device 10 to generate sliding resistance against the movement of the piston 21. The O-ring 28 is an annular elastic part made of an elastic material such as rubber. The inner diameter of the O-ring 28 when no pressure is applied to its inner circumferential surface is slightly smaller than the outer diameter of the nut 18. In this embodiment, an O-ring 28 with a rectangular cross section is used.

The O-ring 28 is positioned next to the cylinder 20. That is, in the linear motion direction of the piston 21, it is further back in the direction R than the rear end of the nut 18 in the initial position, and further forward in the direction F than the rear end of the nut 18 in the most retracted position. Here, the rear end of the nut 18 refers to the end of the nut 18 in the direction R excluding the stopper 18A.

As shown in FIG. 1, when the piston 21 is in its initial position, only the stopper 18A of the nut 18 abuts against the inner circumferential surface of the O-ring 28. When the piston 21 advances a certain distance from its initial position, nothing abuts against the inner circumferential surface of the O-ring 28. As described above, the electric braking device 10 controls the braking force of the wheel 27 by linearly moving the piston 21 in the forward direction F from its initial position. Therefore, during braking force control, the O-ring 28 is in one of two states: one where only the stopper 18A abuts against its inner circumferential surface, or one where nothing abuts against its inner circumferential surface.

Figure 2 shows the state of the cylinder 20 and its surrounding area when the piston 21 is positioned in the backward direction R from its initial position. As shown in Figure 2, when the piston 21 moves in the backward direction R from its initial position, the side of the nut 18 comes into contact with the entire inner circumferential surface of the O-ring 28. In this embodiment, this O-ring 28 corresponds to a resistance generating portion that increases the sliding resistance generated by the movement of the piston 21 when the piston 21 moves in the backward direction R from its initial position compared to when it moves in the forward direction F from its initial position.

<Operation and effect of the first embodiment>
As described above, the electric braking device 10 of this embodiment generates a braking force on the wheels 27 by using the power of the electric motor 11 to move the piston 21 forward from its initial position. If the electric motor 11 loses power due to a power failure or the like while generating this braking force, the piston 21 is pushed back by the hydraulic pressure in the hydraulic chamber 22 and moves backward.

In the electric braking device 10 of this embodiment, when the piston 21 moves in the backward direction R from its initial position, the O-ring 28 abuts against the nut 18. The O-ring 28 absorbs the kinetic energy of the nut 18 through elastic changes. The O-ring 28 also makes sliding contact with the side of the nut 18 and is compressed radially outward due to pressure from the side. The O-ring 28 generates an elastic repulsive force in response to compression, increasing the surface pressure between the contact surfaces of the nut 18 and the O-ring 28. Therefore, the sliding contact of the O-ring 28 with the nut 18 increases the sliding resistance to the linear movement of the nut 18. In other words, when the piston 21 moves in the backward direction R from its initial position, the sliding resistance generated by the movement of the piston 21 is greater than when the piston 21 moves in the forward direction F from its initial position. Therefore, the abutment between the nut 18 and the O-ring 28 reduces the backward speed of the piston 21. This reduces the impact caused by a loss of power from the electric motor 11 while braking force is being generated.

In this embodiment, the O-ring 28, which is the resistance generating part, is positioned radially outside the nut 18 of the linear motion conversion mechanism 13. In other words, the O-ring 28 is installed on the wall surface of the cylinder 20. The location where the O-ring 28 is installed is a location where it does not interfere with the movement of parts other than the nut 18. Furthermore, the location where the O-ring 28 is installed is a location where installing the O-ring 28 does not require any changes to the placement of other parts.

(Modification of the first embodiment)
This embodiment can be modified as follows: This embodiment and the following modifications can be combined and implemented within the scope of technical compatibility.

<About the linear motion conversion mechanism>
The linear motion conversion mechanism 13 in the above embodiment is configured so that the nut 18 moves linearly in response to the rotation of the screw shaft 17. That is, in the above embodiment, the linear motion conversion mechanism 13 is adopted in which the screw shaft 17 is the rotating part and the nut 18 is the linear motion part. Instead of such a linear motion conversion mechanism 13, a screw mechanism in which the nut is the rotating part and the screw shaft is the linear motion part may be adopted as the linear motion conversion mechanism.

The linear motion conversion mechanism 30 of the electric braking device 10 shown in Figure 3 is configured with a screw shaft 31 as the linear motion part and a nut 32 as the rotating part. That is, the linear motion conversion mechanism 30 in Figure 3 has the screw shaft 31 that moves linearly together with the piston 21, and the nut 32 that rotates together with the second gear 16. Note that Figure 3 shows the state of the electric braking device 10 when the piston 21 is in the initial position. When using such a linear motion conversion mechanism 30, it is recommended to install an O-ring 33 at the following position in the cylinder 20, as shown in Figure 3. That is, in the linear motion direction of the piston 21, it is located further backward in the direction R than the rear end of the piston 21 in the initial position, and further forward in the direction F than the rear end of the piston 21 in the most retracted position. Here, the rear end of the piston 21 refers to the end of the piston 21 in the backward direction R.

In addition, instead of a ball screw, a screw mechanism using a sliding screw may be used as the linear motion conversion mechanism. Furthermore, a linear motion conversion mechanism other than a screw mechanism may be used. In such cases, if the screw shaft is the rotating part and the nut is the linear motion part, O-ring 28 may be installed in the same position as in Figure 1. Furthermore, if the nut is the rotating part and the screw shaft is the linear motion part, O-ring 33 may be installed in the same position as in Figure 3.

<About O-rings>
The O-ring 28 may have a cross section other than a rectangular one, such as a circular cross section. Depending on the shape of the O-ring, it may be possible to more efficiently and effectively generate sliding resistance to slow down the retraction of the piston 21. Two examples of such O-ring shapes are described below.

The cross section of the O-ring 40 shown in Figure 4 has an arch shape that protrudes radially outward from the nut 18. The O-ring 40 is installed in a recess 41 formed in the side surface of the cylinder 20. The recess 41 is formed in a curved shape that follows the shape of the radially outer surface of the O-ring 40. In the following description, the end of the O-ring 40 facing the backward direction R will be referred to as the pressure-receiving portion 42, and the end facing the forward direction F will be referred to as the displacement portion 43.

Note that Figure 4 shows the state when the piston 21 is in the initial position. The O-ring 40 is positioned next to the cylinder 20 in the linear direction. In other words, the pressure-receiving portion 42 is positioned further back in the direction R than the rear end of the nut 18 in the initial position, and the displacement portion 43 is positioned further forward in the direction F than the rear end of the nut 18 in the initial position.

When the O-ring 40 is not being pressed from the side of the nut 18, the inner diameter of the displacement portion 43 is larger than the outer diameter of the nut 18. Therefore, when the piston 21 is in the initial position or further forward in the direction F than the initial position, the displacement portion 43 is spaced apart from the side of the nut 18.

On the other hand, when the O-ring 40 is not receiving pressure from the side of the nut 18, the inner diameter of the pressure-receiving portion 42 is smaller than the outer diameter of the nut 18. Therefore, as shown in FIG. 5, when the nut 18 retracts to a position where its side contacts the pressure-receiving portion 42, the side of the nut 18 applies a radially outward pressure to the pressure-receiving portion 42. The elastic deformation of the O-ring 40 in response to this pressure on the pressure-receiving portion 42 is constrained by the shape of the recess 41. Therefore, as shown by the arrow in the figure, the elastic deformation of the O-ring 40 progresses from the pressure-receiving portion 42 toward the displacement portion 43. Furthermore, when the side of the nut 18 presses the pressure-receiving portion 42, the entire O-ring 40 moves along the shape of the recess 41. As a result, the displacement portion 43 protrudes radially inward from within the recess 41 and is pressed against the side of the nut 18. As such, the O-ring 40 has a pressure-receiving portion 42 that comes into sliding contact with the side surface of the nut 18 and is pressed by the side surface when the piston 21 moves backward from its initial position. The O-ring 40 also has a displacement portion 43 that comes into contact with the side surface of the nut 18 as the O-ring 40 is deformed and moved by the pressure-receiving portion 42 caused by the side surface of the nut 18. The O-ring 40 generates sliding resistance against the nut 18 at two locations: the pressure-receiving portion 42 and the displacement portion 43. In other words, the O-ring 40 can generate sliding resistance similar to that generated when two O-rings are arranged side by side in the linear motion direction.

If the O-ring generates sliding resistance when the piston 21 is in the initial position or when it is positioned further forward in the forward direction F than the initial position, this will interfere with the linear movement of the piston 21 for braking control. Therefore, when using two O-rings, both O-rings must be positioned further backward in the backward direction R than the rear end of the nut 18 in the initial position. In this case, only one O-ring can generate sliding resistance until the nut 18 retracts to a position where it contacts the O-ring positioned in the backward direction R. In contrast, the O-ring 40 configured as described above can generate sliding resistance equivalent to that of two O-rings when the nut 18 retracts and the pressure-receiving portion 42 contacts the side surface. Therefore, the retraction of the piston 21 due to a loss of power from the electric motor 11 can be decelerated earlier than when two O-rings are used.

Figure 6 shows a second example of an O-ring shape. The O-ring 44 shown in Figure 6 has a tapered inner surface that is inclined so that it is located radially inside the nut 18 in the backward direction R compared to the forward direction F, and serves as a sliding surface for the nut 18. Furthermore, when not under pressure from the nut 18, the inner diameter of the inner surface of the O-ring 44 at the end in the backward direction R is smaller than the outer diameter of the nut 18. With this O-ring 44, the fit with the nut 18 gradually strengthens as it moves in the backward direction R. Therefore, the O-ring 44 is able to generate a large binding force when compressed, i.e., a large sliding resistance.

In the above embodiment and modified examples, an O-ring, i.e., an annular elastic part, was used to reduce the retraction speed of the piston 21 in the retraction direction R from the initial position. It is also possible to reduce the retraction speed of the piston 21 using an elastic part that is not annular.

Second Embodiment
Next, a second embodiment of the electric braking device will be described in detail with reference to Figures 7 and 8. In this embodiment, components common to the above embodiment will be denoted by the same reference numerals, and detailed description thereof will be omitted.

In the above embodiment, the cylinder 20 was equipped with an elastic component as a resistance generator. In the electric braking device of this embodiment, the threaded portion of the linear motion conversion mechanism is configured as the resistance generator. Note that the electric braking device of this embodiment has the same configuration as the above embodiment, except that the threaded portion of the linear motion conversion mechanism has a different configuration and does not have an elastic component as a resistance generator.

Figure 7 shows the state of the threaded portion of the linear motion conversion mechanism when the piston 21 is in the initial position in the electric braking device of this embodiment. Note that Figure 7 also shows the threaded portions of the linear motion part 50 and the rotating part 51 near the end of the rotating part 51 in the backward direction R. Incidentally, in the case of the linear motion conversion mechanism 13 in Figure 1, the nut 18 corresponds to the linear motion part 50, and the screw shaft 17 corresponds to the rotating part 51. Furthermore, in the case of the linear motion conversion mechanism 30 in Figure 3, the screw shaft 31 corresponds to the linear motion part 50, and the nut 32 corresponds to the rotating part 51.

As shown in Figure 7, a thread groove 53 is formed on the threaded portion of the linear moving part 50. Furthermore, thread grooves 54, 54A are formed on the threaded portion of the rotating part 51. The threaded portion of the linear moving part 50 and the threaded portion of the rotating part 51 are meshed with each other via balls 52 embedded between the thread grooves 53, 54, 54A. As shown in Figure 7, when the piston 21 is in the initial position, there is a portion of the threaded portion of the rotating part 51 in the retraction direction R that is not meshed with the threaded portion of the linear moving part 50. The thread grooves 54A in these portions are shallower than the thread grooves 54 in other portions of the threaded portion of the rotating part 51.

As shown in FIG. 8 , when the piston 21 moves in the backward direction R from the initial position, the portion of the threaded portion of the rotating portion 51 where the shallow thread groove 54A is formed also meshes with the threaded portion of the linear motion portion 50. In the portion where the shallow thread groove 54A is formed, the clearance between the threaded portions, i.e., the axial gap formed between the ball 52 and the thread grooves 53 and 54A, is smaller than in the portion where the deep thread groove 54 is formed. Smaller clearance between the threaded portions increases the rolling resistance of the ball 52. Therefore, when the piston 21 moves in the backward direction R from the initial position, the sliding resistance generated by the movement of the piston 21 increases. Therefore, the threaded portion of the rotating portion 51 formed as described above constitutes a resistance generating portion that increases the sliding resistance generated by the movement of the piston 21 when moving in the backward direction R from the initial position compared to when moving in the forward direction F from the initial position. Therefore, this embodiment also reduces the impact caused by a loss of power from the electric motor 11 while generating braking force. Instead of shallowing the thread groove 54A, the width of the thread groove 54A may be narrowed to reduce the clearance between the threaded portions.

(Modification of the second embodiment)
This embodiment can be modified as follows: This embodiment and the following modifications can be combined and implemented within the scope of technical compatibility.

- In the above embodiment, the threaded portion of the rotating portion 51 was made a resistance generating portion by making the thread groove 54A shallower than the other portions. Instead of making the groove shallower, the threaded portion of the rotating portion 51 can also be made a resistance generating portion by making the surface roughness of the thread groove 54A greater than the surface roughness of the thread groove 54. That is, in the threaded portion of the rotating portion 51, the portion that meshes with the threaded portion of the linear motion portion 50 only when the piston 21 is positioned in the backward direction R from the initial position has a greater surface roughness than the portion that meshes when the piston 21 is positioned in the forward direction F from the initial position. Even in this case, the rolling resistance of the ball 52 is greater in the portion where the thread groove 54A with a greater surface roughness is formed. Therefore, even in this case, when the piston 21 moves in the backward direction R from the initial position, the sliding resistance to the linear motion of the piston 21 is greater.

- Even in a linear motion conversion mechanism that uses a sliding screw instead of a ball screw, the threaded portion of the rotating portion can function as a resistance generating portion. Figures 9 and 10 show the cross-sectional structures of the threaded portions of the linear motion portion 50 and rotating portion 51 of a linear motion conversion mechanism that uses a sliding screw. Figure 9 shows the state of the threaded portion when the piston 21 is located in the initial position. Figure 10 shows the state of the threaded portion when the piston 21 has moved in the backward direction R from the initial position. The threaded portion of the rotating portion 51 is formed with thread grooves 57 and 57A that engage with the threaded groove 56 formed in the threaded portion of the linear motion portion 50. The threaded portion of the rotating portion 51 is formed with thread grooves 57A in a portion that engages with the threaded portion of the linear motion portion 50 only when the piston 21 is located in the backward direction R from the initial position, and is formed in a shallower groove than the thread grooves 57 in other portions. As shown in Figure 10, the clearance formed between the thread groove 56 (the so-called flank gap) is smaller in the portion where the shallow thread groove 57A is formed than in the portion where the deep thread groove 57 is formed. Therefore, when the piston 21 moves in the backward direction R from its initial position, the sliding resistance between the thread grooves 56, 57, and 57A, and therefore the sliding resistance to the linear movement of the piston 21, increases. Alternatively, by making the surface roughness of the thread groove 57A greater than the surface roughness of the thread groove 57, it is possible to increase the sliding resistance to the linear movement of the piston 21 when it moves in the backward direction R from its initial position.

(Third embodiment)
Next, a third embodiment of the electric braking device will be described in detail with reference to Figures 11 and 12. In this embodiment, components common to the above-described embodiment will be denoted by the same reference numerals, and detailed description thereof will be omitted.

As shown in FIG. 11 , the screw shaft 17 and second gear 16 of the electric braking device 10 of this embodiment have through holes 60 that penetrate them in the axial direction. The electric braking device 10 of this embodiment also has a round rod-shaped shaft member 61 installed, one end of which is fixed to the piston 21 and inserted into the through hole 60. Furthermore, a sliding hole 62 is formed in the inner wall of the housing 19 at a portion located in the backward direction R of the second gear 16. The sliding hole 62 is located on an extension of the through hole 60 in the extension direction. The through hole 60 has an inner diameter that is larger than the outer diameter of the shaft member 61.

Figure 12 shows the cross-sectional structure of the sliding contact hole 62 and its surrounding area. In Figure 12, the tip of the shaft material 61 when the piston 21 is in the initial position is shown by a solid line. Furthermore, Figure 12 also shows the tip of the shaft material 61 when the piston 21 has moved in the backward direction R from the initial position by a two-dot chain line. As shown in Figure 12, the inner diameter of the sliding contact hole 62 at the portion where the tip of the shaft material 61 is located when the piston 21 is in the initial position is formed to be larger than the outer diameter of the shaft material 61. On the other hand, the inner diameter of the sliding contact hole 62 at the portion where the tip of the shaft material 61 is located when the piston 21 has moved in the backward direction R from the initial position is formed to be slightly smaller than the outer diameter of the shaft material 61. In this embodiment, this sliding contact hole 62 corresponds to the sliding contact portion that slides against the shaft material 61 when the piston 21 has moved backward from the initial position.

When the piston 21 moves in the backward direction R from the initial position, the tip of the shaft 61 slides against the side wall of the sliding hole 62, generating sliding resistance. Therefore, the shaft 61 and sliding hole 62 form a resistance generating portion that increases the sliding resistance against the linear movement of the piston 21 when the piston 21 is further backward R than when the piston 21 is further forward F than the initial position. Therefore, this embodiment also makes it possible to mitigate the impact caused by a loss of power from the electric motor 11 while braking force is being generated.

When the tip of the shaft 61 is in sliding contact with the side wall of the sliding hole 62, the shaft 61 applies pressure to the side wall of the sliding hole 62 as the piston 21 retracts. If this pressure is applied to moving parts such as the screw shaft 17 or the second gear 16, it could affect the operation of those parts. In this regard, in this embodiment, the sliding hole 62, which serves as the sliding contact point with the tip of the shaft 61, is formed in the inner wall of the housing 19. This reduces the impact of the pressure of the shaft 61 on the operation of the electric braking device 10. In this embodiment, through holes 60 are formed in the screw shaft 17 and the second gear 16, and the shaft 61 is positioned within these through holes 60. As a result, in this embodiment, the sliding hole 62, which serves as the sliding contact point, can be positioned on the opposite side of the screw shaft 17 from the piston 21.

This embodiment can be modified as follows: This embodiment and the following modifications can be combined and implemented within the scope of technical compatibility.
A structure other than a hole may be provided as the sliding contact portion as long as it is capable of sliding contact with the shaft member 61 when the piston 21 moves in the backward direction R from the initial position.

- The sliding contact portion may be provided in a location other than the inner wall of the housing 19, such as the end of the shaft 61 in the forward direction F. If the sliding contact portion is provided in a location where there is no linear motion conversion mechanism between it and the piston 21, the through hole 60 that passes through the screw shaft 17 in the axial direction can be omitted.

DESCRIPTION OF SYMBOLS 10... Electric braking device 11... Electric motor 12... Rotation transmission mechanism 13, 30... Linear motion conversion mechanism 14... Output shaft 15... First gear 16... Second gear 17, 31... Screw shaft 18, 32... Nut 18A... Stopper 19... Housing 20... Cylinder 21... Piston 22... Fluid chamber 23... Input port 24... Output port 25... Reservoir tank 26... Wheel cylinder 26A... Brake shoe 26B... Brake disc 27... Wheel 28, 33, 40, 44... O-ring 41... Recess 42... Pressure receiving portion 43... Displacement portion 50... Linear motion portion 51... Rotating portion 52... Ball 53, 54, 54A, 56, 57, 57A... Thread groove 60... Through hole 61... Shaft material 62... Sliding contact hole

Claims (7)

An electric braking device that transmits rotational motion generated by an electric motor to a linear motion conversion mechanism, converts the rotational motion into linear motion that drives a piston provided inside a cylinder by the linear motion conversion mechanism, and presses a friction part against a frictioned part that rotates together with a wheel of the vehicle, thereby generating a braking force on the vehicle,
a resistance generating section that, when a boundary position between a position of the piston that is normally used and a position of the piston that is not normally used in the electric braking device is defined as an initial position, when the piston moves from the initial position in a backward direction to reduce the braking force of the vehicle, makes the sliding resistance generated in association with the movement of the piston larger than when the piston moves from the initial position in a forward direction to increase the braking force of the vehicle ,
the linear motion conversion mechanism includes a rotating part that is rotated by the electric motor and a linear motion part that moves linearly together with the piston,
the resistance generating portion is disposed radially outward of the linear moving portion, and has an elastic component that comes into sliding contact with a side surface of the linear moving portion when the piston moves in the retreating direction from the initial position, and is compressed radially of the linear moving portion by pressure from the side surface . 2. The electric braking device according to claim 1, wherein the elastic component includes: a pressure-receiving portion that is pressed by the linear-acting portion when the piston moves in the retraction direction from the initial position; and a displacement portion that comes into contact with the linear-acting portion as a result of the pressure-receiving portion being pressed by the linear-acting portion and causing at least one of movement and deformation. The electric braking device according to claim 1 , wherein a sliding surface of the elastic part that comes into contact with the linear moving part is inclined so that a position in the backward direction is located radially inside the linear moving part compared to a position in the forward direction. An electric braking device that transmits rotational motion generated by an electric motor to a linear motion conversion mechanism, converts the rotational motion into linear motion that drives a piston provided inside a cylinder by the linear motion conversion mechanism, and presses a friction part against a frictioned part that rotates together with a wheel of the vehicle, thereby generating a braking force on the vehicle,
a resistance generating section that, when a boundary position between a position of the piston that is normally used and a position of the piston that is not normally used in the electric braking device is defined as an initial position, when the piston moves from the initial position in a backward direction to reduce the braking force of the vehicle, makes the sliding resistance generated in association with the movement of the piston larger than when the piston moves from the initial position in a forward direction to increase the braking force of the vehicle ,
the linear motion conversion mechanism is a screw mechanism in which one of the nut and the screw shaft is a rotating part that is rotated by the electric motor, and the other is a linear motion part that moves linearly together with the piston,
the resistance generating section is configured such that the clearance between the threaded portion of the rotating section and the threaded portion of the linear moving section is smaller at a portion where the threaded portion of the rotating section and the threaded portion of the linear moving section mesh together only when the piston moves backward from the initial position than at a portion where they mesh together when the piston moves forward from the initial position . An electric braking device that transmits rotational motion generated by an electric motor to a linear motion conversion mechanism, converts the rotational motion into linear motion that drives a piston provided inside a cylinder by the linear motion conversion mechanism, and presses a friction part against a frictioned part that rotates together with a wheel of the vehicle, thereby generating a braking force on the vehicle,
a resistance generating section that, when a boundary position between a position of the piston that is normally used and a position of the piston that is not normally used in the electric braking device is defined as an initial position, when the piston moves from the initial position in a backward direction to reduce the braking force of the vehicle, makes the sliding resistance generated in association with the movement of the piston larger than when the piston moves from the initial position in a forward direction to increase the braking force of the vehicle ,
the linear motion conversion mechanism is a screw mechanism in which one of the nut and the screw shaft is a rotating part that is rotated by the electric motor, and the other is a linear motion part that moves linearly together with the piston,
In a portion where the threaded portion of the rotating portion and the threaded portion of the linear motion portion mesh together only when the piston moves backward from the initial position, the resistance generating portion is configured by making the surface roughness of the threaded portion of the rotating portion larger than that of a portion where the threaded portion meshes together when the piston moves forward from the initial position . An electric braking device that transmits rotational motion generated by an electric motor to a linear motion conversion mechanism, converts the rotational motion into linear motion that drives a piston provided inside a cylinder by the linear motion conversion mechanism, and presses a friction part against a frictioned part that rotates together with a wheel of the vehicle, thereby generating a braking force on the vehicle,
a resistance generating section that, when a boundary position between a position of the piston that is normally used and a position of the piston that is not normally used in the electric braking device is defined as an initial position, when the piston moves from the initial position in a backward direction to reduce the braking force of the vehicle, makes the sliding resistance generated in association with the movement of the piston larger than when the piston moves from the initial position in a forward direction to increase the braking force of the vehicle,
The resistance generating portion has a shaft that moves linearly together with the piston, and a sliding contact portion that comes into sliding contact with the shaft when the piston moves in the backward direction from the initial position . the linear motion conversion mechanism is a screw mechanism having a screw shaft rotated by the electric motor and a nut that moves linearly together with the piston,
The screw shaft has a through hole that penetrates in the axial direction,
The sliding contact portion is disposed on the opposite side of the screw shaft from the piston,
The shaft member is provided in the through hole and is disposed between the sliding contact portion and the piston.
7. The electric braking device according to claim 6 .